Liquid-liquid extraction (LLE) using sulfolane with water as the extractive solvent is the most important commercial process for purifying the full-range (C6-C8) of aromatic hydrocarbons from petroleum streams, including reformate, pyrolysis gasoline, coke oven oil, and coal tar. U.S. Pat. No. 3,179,708 to Penisten describes an early sulfolane solvent based LLE process that employed an LLE column, a raffinate water washing column (WWC), and a solvent recovery column (SRC). A hydrocarbon feed mixture is contacted in the LLE column with an aqueous sulfolane solvent, which selectively dissolves the aromatic components from the hydrocarbon feedstock, to form a raffinate phase comprising one or more non-aromatic hydrocarbons and an extract phase comprising the solvent and at least one dissolved aromatic compound. The extract phase is transferred to the SRC where the aromatic hydrocarbons are steam stripped from the sulfolane solvent, thereby recovering the components of greatest volatility from the overhead and the purified aromatic product from the side-cut of the SRC. The overhead light components which include aromatics are recycled as a part of the reflux to the LLE column. Finally, water condensate collected from the SRC overhead and side-cut are combined and recycled to the WWC where sulfolane is removed from the raffinate phase to produce a solvent free non-aromatic product.
U.S. Pat. No. 4,046,675 to Asselin disclose a crucial improvement to the earlier LLE processes by incorporating an extractive stripping column (ESC) to remove non-aromatic contaminants from the extract phase of the LLE column before entering the SRC. Non-aromatic components and a significant portion of aromatic components in the LLE extract phase are removed from the ESC overhead and recycled as a liquid reflux to the LLE column. Rich solvent, which contains aromatic components and is virtually free from non-aromatic components, is withdrawn from the bottom of the ESC and fed to the SRC. To enhance ESC operations, an aromatic-containing rich solvent is withdrawn from the SRC side-cut and introduced into the ESC with the LLE extract phase.
The addition of the ESC has been critical to the success of the current LLE process for recovering the full range (C6-C8) aromatic hydrocarbons using aqueous sulfolane as the extractive solvent. However, a major drawback of this process is the high energy (steam) consumption of the ESC, in which all the overhead condensate is recycled to the lower portion of the LLE column as the reflux. In order to maintain the purity of the aromatic product, a substantial amount of energy is needed by the reboiler to vaporize and remove nearly all non-aromatic hydrocarbons from the bottom of the ESC. As a result of this requirement, the overhead vapor from the ESC can contain as much as 25-30% benzene and nearly 10% heavier aromatics, which are condensed and recycled to the bottom of LLE column as the reflux. Consequently, the recycled benzene and heavier aromatics are extracted by the solvent again in the LLE column and fed back to the ESC. Another significant drawback of current ESC operations is that light non-aromatic hydrocarbons (C5-C6), due their higher affinities with the solvent, are continuously accumulated in a closed loop between the top of ESC and the bottom of the LLE column with no way out but consuming a significant amount of vaporization energy. Therefore, this stream has to be purged from time to time in order to keep the process in continuous operation. This large reflux operation not only requires high energy but also creates a bottleneck at the ESC and reduces throughput of the LLE process.
U.S. Pat. No. 5,336,840 to Forte notes that in 1986, the energy costs (comprising steam, electric power and cooling water) for a typical 10,000 barrel per day (or 420,000 metric ton per year) sulfolane solvent based LLE process, amounted to as high as 83%, with solvent make-up charges, labor and maintenance costs making up for the remaining 17% of the total processing costs. In light of the recent drastic increases in oil and natural gas prices, energy costs associated with this process today are significantly higher, thus any reduction in processing energy would be very beneficial.
Various schemes have been proposed and developed with the goal of generating energy savings in the basic process of continuous liquid-liquid extraction for aromatic hydrocarbon recovery and steam stripping for solvent recovery. Most of these schemes which are based on heat integration, such as by using heat exchangers between process streams, pressure reduction devices between process vessels, and the like, have achieved only limited success but with significant increase in equipment costs.